In recent years, there has been a dramatic increase in the demand for propylene to feed the growing markets for polypropylene, propylene oxide and acrylic acid. Currently, most of the propylene produced worldwide (74 million tons/year) is a by-product from steam cracking units (57%) which primarily produce ethylene, or a by-product from Fluid Catalytic Cracking (FCC) units (30%) which primarily produce gasoline. These processes cannot respond adequately to a rapid increase in propylene demand.
Raffinate is the residue C4 stream from a naphtha cracking process or from a gas cracking process when components are removed (the C4 stream typically containing, as its chief components, n-butane, 1-butene, 2-butene, isobutene and 1,3-butadiene, and optionally some isobutane and said chief components together forming up to 99% or more of the C4 stream). Specifically, Raffinate-2 is the C4 residual obtained after separation of 1,3-butadiene and isobutylene from the C4 raffinate stream and consists mainly of cis- or trans-2-butene, 1-butene, and n-butane. Similarly, Raffinate-3 is the C4 residual obtained after separation of 1,3-butadiene, isobutylene, and 1-butene from the C4 raffinate stream and consists mainly of cis- or trans-2-butene, n-butane, and unseparated 1-butene. Utilizing Raffinate-2 and Raffinate 3 streams for conversion to propylene is desirable to increase the available supply of propylene.
Development of a metathesis catalyst necessary to convert Raffinate-2 and Raffinate 3 streams have relied on wet impregnation or grafting techniques to insert a metal oxide into a previously synthesized support material. However, wet impregnation or grafting techniques are limited in that the entire particle volume of the catalyst is not always accessible for insertion of a metal oxide and control of the properties of the resulting metathesis catalyst is limited.